1. Field of the Invention
The present invention generally relates to thermally decomposable organometallic source reagents that are useful in chemical vapor deposition (CVD) processes, for the formation of metal films on substrates. More specifically, the invention relates to Group II .beta.-diketonate tetrahydrofuran complex source reagents useful for liquid delivery chemical vapor deposition of Group II metal-containing films.
2. Description of the Related Art
Chemical vapor deposition is widely used for the formation of metal films and/or metal containing films on a variety of substrates. CVD is a particularly attractive method for forming metal films because it is readily scaled up to production runs and because the electronics industry has a wide experience and an established equipment base in the use of CVD technology which can be applied to CVD processes.
CVD requires source reagents that are sufficiently volatile to permit their gas phase transport into the decomposition reactor. The source reagent must decompose in the CVD reactor to deposit only the desired element(s) at the desired growth temperature on the substrate. Premature gas phase reactions are desirably avoided, and it generally is desired to controllably deliver source reagents into the CVD reactor to effect correspondingly close control of film stoichiometry.
Many potentially useful metals do not form compounds that are well suited for CVD. Although some source reagents are solids that are amenable to sublimation for gas-phase transport into the CVD reactor, the sublimation temperature may be very close to the decomposition temperature. Accordingly, the reagent may begin to decompose in the lines leading to the CVD reactor, and it then becomes difficult to control the stoichiometry of the deposited films.
Accordingly, there is a continuing search in the art for improved source reagent compositions which are more amenable to vaporization to form the source component vapor for CVD processes, for applications such as the formation of diffusion barriers, conductors, dielectrics, protective coatings, phosphors, electroluminescent structures, ferroelectrics, giant magnetoresistive films, corrosion-resistant films, and mixed metal films.
In the chemical vapor deposition of multicomponent material systems, multiple source reagents are delivered to the CVD reactor. A particularly advantageous way of delivering multiple source reagents is to accurately mix neat liquid source reagents or liquid solutions of source reagents and then flash vaporize the mixture and deliver the resulting vapor to the reactor for deposition of metal components on a substrate heated to an appropriate temperature (liquid delivery metalorganic chemical vapor deposition). It is possible in this situation for the reagents to undergo reactions, either in the liquid phase before vaporization or in the gas phase after vaporization. If these reactions convert a source reagent to an insoluble or non-volatile product, or to a material of different chemical or physical properties, then the elements contained in that product will not reach the substrate and the stoichiometry of the deposited film will be incorrect.
Examples of this problem (wherein Et is ethyl; tBu is tert-butyl; iPr is isopropyl; and thd is tetramethylheptanedionate) include the following:
(i) during deposition of PbZr.sub.x Ti.sub.1-x O.sub.3, using (Et).sub.4 Pb, Zr(OtBu).sub.4, and Ti(OiPr).sub.4 source reagents, ligand exchange between the Zr and Ti reagents resulted in formation of Zr(OiPr).sub.4 (and perhaps other products of which Zr(OiPr).sub.4 is a monomer), which had very low volatility and which condensed in the gas manifold or vaporizer; PA1 (ii) when solutions of Ba(thd).sub.2 and Ti(OiPr).sub.4 were mixed prior to vaporization, an insoluble precipitate was formed, presumably Ba(OiPr).sub.2 or the mixed alcoxide .beta.-diketonate of Ti , Ti(OiPr).sub.2 (thd).sub.2, was found; and PA1 (iii) when solutions of Pb(thd).sub.2 and Ti(OiPr).sub.4 were mixed in butyl acetate, the reagents reacted to form compounds of differing physical properties, such as Pb(OiPt).sub.2 and Ti(OiPr).sub.2 (thd).sub.2. PA1 providing liquid precursors for each of the barium, strontium and titanium components of the BST film; PA1 vaporizing each of the liquid precursors, separately or all together, to form the corresponding precursor vapor; and PA1 contacting the precursor vapor with a substrate to deposit barium, strontium and titanium thereon;
Another specific example illustrating this problem is the preparation of films of strontium bismuth tantalate and strontium bismuth niobate (SrBi.sub.2 Ta.sub.2 O.sub.9 and SrBi.sub.2 Nb.sub.2 O.sub.9) by CVD for use in non-volatile ferroelectric random access memories. The most commonly used strontium source reagents are .beta.-diketonate complexes such as Sr(thd).sub.2. When a solution is heated containing the following source reagents for deposition of SrBi.sub.2 Ta.sub.2 O.sub.9 : EQU Sr(thd).sub.2 ; Ta(OEt).sub.5 ; and Bi(Ph).sub.3 wherein Ph=phenyl,
the ethoxide ligands of the tantalum reagent exchange with the thd ligands of the strontium reagent, leading to the formation of undesirable strontium alkoxide species that have reduced volatility and that can decompose in the vaporization zone. Alternatively, when these reagents are provided separately in bubblers, similar ligand exchange reactions occur in the gas phase; the resulting solids constrict the gas lines, alter the film stoichiometry, and/or lead to the formation of particles in the films.
In certain instances, such problems can be avoided by using identical ligands on the metals to make ligand exchange a degenerate reaction (i.e., where the exchanging ligand is identical to the original ligand). Examples of this approach include the use of tetraethylorthosilicate, triethylborate and triethylphosphite for deposition of borophosphosilicate glasses (J. Electrochem. Soc., 1987, 134(2), 430). In many instances, however, this method for avoiding the problem is not possible because the appropriate compound does not exist, is too unstable or involatile to be used for CVD, or otherwise has disadvantageous physicochemical material properties. For example, for deposition of PbZr.sub.x Ti.sub.1-x O.sub.3, a reagent system with identical ligands is problematic because while Pb(thd).sub.2 and Zr(thd).sub.4 are stable and volatile, Ti(thd).sub.4 does not exist and Ti(thd).sub.3 is extremely air sensitive. Similarly, while Ti(OtBu).sub.4 and Zr(OtBu).sub.4 are stable and volatile, Pb(OtBu).sub.2 is oligomeric and thermally unstable at temperatures required for volatilization.
The foregoing problems are also encountered in the circumstance where the metal source reagent is provided in a liquid solution and the solvent contains moieties that react with ligands of the source reagent compound to produce undesirable ligand exchange reaction by-products that display different physical properties and are involatile or insoluble in organic solvents.
As a result of interest in barium and/or strontium-based oxide thin films having desirable electrical properties, including barium strontium titanate (BST) as a dielectric thin film material, and strontium bismuth tantalate (SBT) as a ferroelectric thin film material, corresponding interest has been focused on liquid delivery MOCVD precursor source reagents for barium and/or strontium.
In this effort of developing new and improved Ba and Sr metalorganic precursors for the thin-film deposition of barium and/or strontium, the main focus of recent research efforts has been directed towards the formation of liquid complexes of Ba and Sr, since the existing precursors for these metals are believed to cause particle formation, resulting in vaporizer or delivery tube clogging and particles in the deposited films. As a result, such existing Ba and/or Sr precursors do not fully satisfy the requirements of the CVD process. Due to the nature of the liquid delivery MOCVD process, however, changes in both the Ba and Sr precursor chemistry are required.
In liquid delivery process applications in which solvent is employed to form a precursor composition, the solvent is often overlooked as a critical chemistry component. However, the solvent is exceedingly important with respect to the delivery and vaporization phenomena, being a major constituent of the chemical solution forming the source reagent composition in such instances, e.g., where the source reagent compound or complex is not utilized as a neat liquid, but rather is dissolved or suspended in a suitable and chemically compatible solvent medium (single- or multi-component).
Designing Group II liquid or low melting point solid CVD precursors represents a significant challenge. For example, the difficulties in obtaining suitable barium source reagent complexes stem from the highly electropositive character of barium, with its large ionic radius. The latter requires high coordination numbers that have to be satisfied by mostly neutral ligands. These ligands cannot coordinate strongly to the electropositive Ba center. As a result, barium CVD precursors are frequently characterized by insufficient thermal stability.
The lability of metal-ligand bonds in barium-based metalorganic compositions coupled with the large size of the metal center result in a tendency to aggregate to form multinuclear species. This can happen either during synthesis or transport, affecting the precursor volatility, or during the CVD process, lowering the transport rate and film growth efficiency. Formation of multinuclear species results in decreased solubility that can have critical impact on the nature and efficiency of the liquid delivery CVD process and may lead to particle formation in thin film growth.
In an actual approach to such precursor design issues, attempts should be made to achieve a best compromise between the basic requirements of stability, volatility and solubility, and particularly between stability and volatility. Previous work has shown that it is difficult to form very stable and simultaneously highly volatile complexes of Ba and Sr, however an appropriate compromised balance of stability and volatility for a given precursor may be utilized to provide an improved CVD process.
Previous film growth research has indicated that barium metalorganic CVD precursors are desirably (a) mononuclear metalorganic complexes, (b) soluble in organic solvents, such as aliphatic hydrocarbons, and (c) fluorine-free. The desired volatility of the complex may be achieved by using appropriate ligands species and limiting inter-nuclear interactions. This is especially critical for large electropositive metal centers such as barium.
As a result of the foregoing, interest in liquid sources of Group II (Ba, Sr, Ca, Mg) precursors has increased, both as a means for reducing the formation of solid particles during the liquid delivery process, and as a means for achieving improved vaporization characteristics. Due to the relatively large radii of the Group II elements, adducts become essential for forming mononuclear species possessing efficient vaporization and gas-phase transport characteristics.
Accordingly, it is an object of the present invention to provide improved Group II metal source reagent compositions for forming corresponding Group II metal-containing films via liquid delivery CVD processes.
It is another object of the invention to provide an improved method of liquid delivery metalorganic chemical vapor deposition utilizing such precursors.
Other objects and advantages of the invention will be more fully apparent from the ensuing disclosure and appended claims.